Turbomachines (such as gas and steam turbines) always have a flow duct for passage of a fluid therethrough. The flow duct, also referred to as “annular space,” is radially inwardly bounded by the shaft of a rotor and radially outwardly by a casing. As used herein, and unless otherwise stated, the terms “radial,” “axial,” and “circumferential direction,” as well as terms derived therefrom, are taken with respect to an axis of rotation of the rotor.
A turbomachine has airfoil arrays (commonly also referred to as “blade rings” and “vane rings) arranged in its annular space. The airfoil arrays include stator vanes or rotor blades which are arranged in succession and substantially equally spaced in the circumferential direction, as well as associated platforms, which are also referred to as “shrouds” and generally have a leading platform edge and a trailing platform edge. These platform edges delimit the platform surface in the axial direction. The term “platform surface” as used herein refers to the surface of the platform that faces the blades or vanes (i.e., the airfoils).
The platform edge that is first passed over by the (axial) primary flow that, during operation, is directed through the annular space of the turbomachine, is referred to herein as “leading” platform edge; the opposite edge is accordingly referred to as “trailing” platform edge. Correspondingly, the terms “downstream” and “upstream” refer to the axial primary flow direction, and, more specifically, only to the axial position, regardless of any possible offset in the circumferential or radial direction. Specifically, a point is understood herein as being located “downstream of the leading edges” (or as being located “downstream of another point”), if, relative to a direct connection between the leading edges (to one another) (or relative to another point), it is axially offset in the direction of (i.e., following) the primary flow. This applies analogously to the term “upstream” (in the opposite direction).
The section of the platform surface that is axially bounded by the direct connections between the leading and trailing edges of adjacent airfoils at the platform surface (i.e., the connections extending in the circumferential direction without deviating axially therefrom) and circumferentially by the pressure side of one airfoil and the suction side of the other airfoil is referred herein as “inter-airfoil strip.” The width of the inter-airfoil strip in the circumferential direction at the leading edges is referred to as “pitch spacing” (of the airfoil array or of an airfoil array segment or of the airfoils). Specifically, the pitch spacing may be measured as the circumferential spacing between the leading edges of adjacent airfoils in the area of the platform surface. The spacing between the leading and trailing edges of the airfoils measured (solely) in the direction of the designated axial primary flow is referred to as the “axial chord length” or “axial chord.”
The pressure side of one airfoil and the suction side of an adjacent airfoil each circumferentially bound what is generally referred to as an airfoil passage. Within the turbomachine, this airfoil passage is radially bounded by what is known as endwalls. These endwalls are formed, on the one hand, by the platforms and, on the other hand, by sections located radially opposite these platforms. In the case of rotor blades, such an opposite endwall is a radially outer section (such as a section of an outer shroud of the rotor blade(s); in the case of stator vanes, it is a radially inner section (such as a radially inner platform of the stator vane(s) or a wall of another stator in the region of the hub).
A fluid flow passing through a flow duct is always affected by the surfaces of the endwalls. Due to their lower velocity, flow layers near these surfaces are deflected to a greater degree than flow layers which are further away from the endwalls. Thus, a secondary flow is formed which is superimposed on an axial primary flow and leads, in particular, to vortices and pressure losses.
To reduce secondary flows, often, contours in the form of elevations and/or depressions are formed in the endwalls.
A variety of such contours, commonly known as “endwall contours,” are known from the prior art.
European Patent Publication EP 1 967 694 A2, for example, shows an elevation in an endwall contour for a turbomachine where an airfoil is on one side partially rooted to the elevation (i.e., a portion of a boundary line between the endwall and the airfoil runs on the elevation).
U.S. Pat. No. 8,511,978 B2 describes an endwall contour where a platform between two airfoils has a depression that is located at least 30% of the axial chord downstream of the leading edges of the airfoils.
The Applicant's patents or patent applications EP 2 487 329 B1, EP 2 787 172 A2, and EP 2 696 029 B1 should also be mentioned by way of example.